A conventional ultrasonic sensor has, for example, a sensor body with an ultrasonic transducer installed therein attached to the bumper of a vehicle. Ultrasonic waves are generated by the ultrasonic transducer and, by receiving reflection waves of the ultrasonic waves, an obstacle existing near the bumper is detected.
FIG. 17 is a partial cross section showing a structure of fixing the conventional ultrasonic sensor 100 to a bumper 101. As shown in the diagram, a sensor body 102 of the ultrasonic sensor 100 has a circular column shape. By inserting the sensor body 102 in a circular hole 101a formed in the bumper 101 from the outside of the bumper 101, the sensor body 102 is fixed to the bumper 101. More practically, a flange 102a is provided at the rear end in the direction of insertion to the bumper 101 in the sensor body 102, and a V-shaped metal spring 103 is provided around the outer periphery of the sensor body 102. Consequently, when the sensor body 102 is inserted in the hole 101a in the bumper 101, in a state where movement in the insertion direction of the sensor body 102 is regulated by the flange 102a, the widening force in the radial direction of the sensor body 102 generated by the metal spring 103 is applied to the wall face constructing the hole 101a of the bumper 101, so that the sensor body 102 is firmly fixed to the bumper 101 (refer to, for example, U.S. Pat. No. 6,759,950).
In recent years, the ultrasonic sensor is becoming intelligent, and a circuit board on which not only an ultrasonic transducer but also a signal processing circuit and the like are formed is being housed in the sensor body. The sensor body is accordingly enlarged, and a problem occurs such that the sensor body cannot be inserted in the hole in the bumper from the outside of the bumper. Therefore, by a structure different from a conventional structure, the ultrasonic sensor has to be attached to the bumper.
To address the above-described problem, the inventors of the present invention have devised a structure in which a bezel (cover) formed in a cylindrical shape and having a flange at its one end is provided. The bezel is fixed by being inserted from the outside of the bumper into a hole in the bumper. After that, by inserting a circular column portion in which an ultrasonic transducer is disposed in the sensor body into the hollow in the bezel from the inside of the bumper (that is, in the direction opposite to the insertion direction of the bezel to the hole in the bumper), the sensor body is fixed to the bumper via the bezel.
In the case of such a structure, the large sensor body has to be held by the bezel functioning as a fixing member. Consequently, the bezel has to be firmly fixed to the bumper, and the sensor body has to be firmly fixed to the bezel.
The above-described problem also arises when the ultrasonic sensor is attached to a part of the vehicle that is different from a bumper in a vehicle body.